Correlation of microstructure and tempered martensite embrittlement in two 4340 steels
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IN T R O D U C T I O N
R E C E N T studies of tempered martensites in ultrahigh strength steels emphasize the importance of correlating microstructure and fracture toughness. For example, the investigation of the effects of chemistry, heat treatment, and microstructure on the fracture behavior of quenched and tempered AISI 4340 steels has been the subject of many research efforts, tl-~l Specifically, the intentions have been to document the micromechanics of fracture at sharp crack tips and at blunt notches and to correlate these microfracture processes with microstructure and macroscopic fracture toughness. It has been shown that when these steels are tempered near 350 ~ a loss in toughness, called tempered martensite embrittlement (TME), is observed. This usually is characterized by a trough in the plot of Charpy impact energy as a function of tempering temperature and by the ductile-brittle transition temperature exhibiting a maximum at the temperature corresponding to the minimum in the Charpy toughness.t9-~21 Various models have been proposed, attempting to provide an explanation for the tempered martensite embrittlement. Several researchers ~13-~7~have found that the drop in toughness can be correlated with the decomposition of lath-boundary-retained austenite and the subsequent formation of interlath cementite films during tempering. These interlath carbides may provide crack nucleation sites or easy crack paths; thus, the fracture path is intergranular with respect to tempered martensite but transgranular with respect to the prior austenite. Recently, the effects of impurity content and grain size on the percentage of intergranular fracture in high strength steels tempered in the TME range have been discussed.
S. LEE and D.Y, LEE are Assistant Professors, Department of Materials Science and Engineering, Pohang Institute of Science and Technology, Pohang 790, Korea. R.J. ASARO is Professor, Division of Engineering, Brown University, Providence, RI 02912. Manuscript submitted June 1, 1988. METALLURGICAL TRANSACTIONS A
Briant and Banerji t11] suggested a mechanism for TME in which impurity elements, primarily phosphorus and sulfur, segregate to the grain boundaries during austenitization. Thus, even in the quenched structure, the prioraustenite grain boundaries are weakened somewhat, and some intergranular fracture may be observed. Materkowski and Krauss t18j also have reported that the fracture mode changes from transgranular to intergranular when the phosphorus content of the 4340 steels is increased from 0.003 to 0.03 wt pct. The present work describes a study of the effect of microstructure on fracture toughness values in two special steels corresponding to AIS14340 composition. One of these steels was aluminum-killed, and the other was deoxidized with titanium-aluminum additions. The purpose of using these deoxidizing treatments was to allow a comparative study of high temperature heat treatments that produce microstructures which lead to large increases in grain size with those which do not. In or
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